SBC won't idle

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Help! The SBC in my 35 Ford suddenly won't idle in gear. Started at Goodguys @ Charlotte last month. Compression is good 175 - 195 pounds. Vacuum is terrible at idle (8 inches). I've changed plugs and wires; pinched off all the vacuum hoses (no difference); swapped carbs (no difference); took the intake manifold off and put all new gaskets on (no difference); dist vacuum advance works; power brakes work. Motor is a mild 355, Comp Cam (280), roller rockers, 600 CFM Edelbrock carb & performer intake. From what I've read the low vacuum reading indicates a manifold leak (doubtful since I just re-mounted it) or late valve timing. From what I've read it is unlikely for the timing chain to "jump time" and still have a running motor.
Any suggestions out there???
Thanks

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You have checked the right things so far. There is one question I have for you though, is your car an automatic? If it is, it might be worth a shot to crawl under it & check where your vacume line hooks up to your vacume modulator. Gary 4T950 Chevy Guy

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Thanks to everyone for your ideas. The cam lobes appear to be OK (watching the valves while I turn the crank pulley). The timing chain and gears are OK too. I guess the next thing I'll do is pull the heads. Guess I'll be trying to fix this motor instead of chopping the top this winter

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Good cam, good compression, no vacuum...new intake gaskets, new carb. i guess you've checked the timing after you pulled the intake
How does it run on the highway? at WOT?

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Timing was OK - 10 degrees BTDC. Vacuum is steady so I don't really think its a valve problem (but I'm sure as hell not an expert or I wouldn't be asking for help)I have a comp 280 cam (never checked the vacuum until I had an idle problem so I don't know how much it made when it was running OK). It ran well on the way back from Charlotte (70 - 75 mph) but wouldn't idle worth a crap in gear. We advanced the timing a lot in Charlotte before I left so it would at least idle some, had some spark knock under load however (premium gas with 10.5/1 CR). One of my friends (with the most experience) thinks I have a problem with the distributor. I'm still going to yank the motor and put a 500 over stock torque converter in it. Hope I stumble into a fix along the way.
Thanks for your input.

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hummmm ...happened at a goodguys event. maybee christopher titus has something to do with it. my bet is a broken valve spring or a stuck valve. did you check the compresion on all cyinders? do a running compression check on each cylinder thats how to spot a broken spring or hung valve.

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hummmm ...happened at a goodguys event. maybee christopher titus has something to do with it. my bet is a broken valve spring or a stuck valve. did you check the compresion on all cyinders? do a running compression check on each cylinder thats how to spot a broken spring or hung valve.

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Compression is pretty good: lowest cylinder is 175 and highest is 197. I've decided to yank it out and take it apart. I didn't really like the 280 cam all that much and I need a higher stall torque converter etc. etc. So instead of chopping it 3 inches I'm going to try to get it to run better.....I might even get around to putting the 10 bolt rear end in it.....the '56 Chev rear is too wide with my vintage ET's and I want to get the back end of it down.

Member

hummmm ...happened at a goodguys event. maybee christopher titus has something to do with it. my bet is a broken valve spring or a stuck valve. did you check the compresion on all cyinders? do a running compression check on each cylinder thats how to spot a broken spring or hung valve.

Click to expand...

Compression is pretty good: lowest cylinder is 175 and highest is 197. I've decided to yank it out and take it apart. I didn't really like the 280 cam all that much and I need a higher stall torque converter etc. etc. So instead of chopping it 3 inches I'm going to try to get it to run better.....I might even get around to putting the 10 bolt rear end in it.....the '56 Chev rear is too wide with my vintage ET's and I want to get the back end of it down. I can always chop it next year.....

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Hey VA,how about more history Ie,it was running fine you parked it got back in it wouldn't idle,you romped on it hard and it wouldn't idle after that,you were in a burnout contest and it wouldn't idle after that ,or what.

A car that has been running fine and then thrashed and suddenly won't idle,particularly if it hooks hard at the end of a thrashin' may very well have just broken the sprague in the torque converter.When the sprague breaks(rolls over)especially in a low or no stall converter it will act a lot like a stick car that the clutch won't disengage in when you put it in gear the car lurches more than normal and then dies.

When you say intake vacuum is low is that in or out of gear?Oh and asked earlier does the car do well at speed?
T.OUT

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What ignition setup are you using? (points, HEI, Pertronix, etc.?) Check voltage in the ignition system and all your ground connections especially since it "just happened" fairly quick - low voltage can cause all sorts of odd symptoms like this...

Member

Hey VA,how about more history Ie,it was running fine you parked it got back in it wouldn't idle,you romped on it hard and it wouldn't idle after that,you were in a burnout contest and it wouldn't idle after that ,or what.

A car that has been running fine and then thrashed and suddenly won't idle,particularly if it hooks hard at the end of a thrashin' may very well have just broken the sprague in the torque converter.When the sprague breaks(rolls over)especially in a low or no stall converter it will act a lot like a stick car that the clutch won't disengage in when you put it in gear the car lurches more than normal and then dies.

When you say intake vacuum is low is that in or out of gear?Oh and asked earlier does the car do well at speed?
T.OUT

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It ran fine all the way from SE VA to Charlotte (about 300 miles). I wasn't driving hard, just keeping up with the other guys. I really noticed the problem the next day while driving to the speedway from the motel. I hadn't thrashed it any (although I do like to do burnouts on a regular basis ) Interesting idea about the torque converter as I have no idea how old it is (but it has been thrashed by me...lots of 6500 RPM 1st gear burnouts). The car ran OK at speed all the way back home. The last time I drove it (a couple of weeks ago) it didn't accelerate well (the motor wouldn't "wind up" like it usually would). As for the vacuum I get 8" at idle in neutral (around 1100 RPM). Normal idle in drive with the brake on was around 650 RPM. If I blip the throttle it jumps up to about 20" on the way back down to idle (very briefly). My power brakes still work OK (always did) but according to one of my friends who put a timing light on the car the timing doesn't advance very well when you give it the gas; we put a vacuum pump on the vacuum advance and it took around 13' to move it. All in all most peculiar problem. Thanks for the input and I'll add a post if/when I figure it out. (I'm being punished for trying to sell it to get a '34 highboy)

Member

What ignition setup are you using? (points, HEI, Pertronix, etc.?) Check voltage in the ignition system and all your ground connections especially since it "just happened" fairly quick - low voltage can cause all sorts of odd symptoms like this...

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Got an HEI. Everything "appears" to be OK but I may try another distributor I know is good after I get the thing back together. Thanks for the input.